In recent years, a light emitting element using a light emitting organic compound has been actively researched and developed. A basic structure of this light emitting element is that which is formed by sandwiching a layer containing a light emitting organic compound between a pair of electrodes. By applying a voltage to this element, electrons and holes are separately injected from the pair of electrodes into the layer containing a light emitting organic compound, and current flows. Then, recombination of these carriers (the electrons and holes) causes the light emitting organic compound to form an excited state and to emit light when the excited state returns to a ground state. Owing to such a mechanism, such a light emitting element is referred to as a current-excitation light emitting element.
Note that excited states an organic compound forms can be a singlet excited state or a triplet excited state. Light emission from the singlet excited state is referred to as fluorescence, and light emission from the triplet excited state is referred to as phosphorescence.
A great advantage of such a light emitting element is that the light emitting element can be manufactured to be thin and lightweight because the light emitting element is formed of an organic thin film with, for example, a thickness of approximately 0.1 μm. In addition, extremely high response speed is another advantage, because time between carrier injection and light emission is approximately 1 μsec or less. These characteristics are considered suitable for a flat panel display element.
Such a light emitting element is formed in a film shape. Thus, surface emission can be easily obtained by forming a large-area element. This characteristic is hard to be obtained by a point light source typified by an incandescent lamp or an LED or a line light source typified by a fluorescent lamp. Therefore, the above described light emitting element also has a high utility value as a surface light source which is applicable to lighting or the like.
Such a light emitting element has many material-dependent problems in improving its element characteristics, and improvement in an element structure, development of materials, and the like are conducted to overcome the problems.
As one of the causes of deterioration of the current-excitation light emitting element, deterioration of a material contained in a layer containing a light emitting substance formed between a pair of electrodes is given. Due to current flow in the layer containing a light emitting substance in the current-excitation light emitting element, the material contained in the layer containing a light emitting substance is repeatedly subjected to oxidation reaction and reduction reaction. When a material which is easily decomposed by oxidation reaction and reduction reaction is contained in the layer containing a light emitting substance, the material is gradually deteriorated by the repeated oxidation reaction and reduction reaction and the light emitting element itself is also deteriorated. Thus, development of an electrochemically stable substance is demanded.
Reference 1 discloses trisarylaminobenzene as a substance with few electrochemical changes (Reference 1: Japanese Patent No. 3419534). However, characteristics such as heat resistance are not sufficient yet, and development of an organic compound with better heat resistance is demanded.